Evolution of Sand-Dominant Subaerial Phase, Atchafalaya Delta, Louisiana

Abstract

In 1972 the Atchafalaya River delta of south-central Louisiana evolved into a subaerial form, which is now prograding at a rapid rate. This latest stage of development has focused attention on one of the most dynamic geologic events of this century in coastal Louisiana. The delta has resulted from (a) steady diversion and capture of Mississippi River flow by the hydraulically more efficient Atchafalaya River, (b) progressive and rapid filling of the Atchafalaya basin, (c) initiation of a subaqueous delta when clay deposition increased significantly in Atchafalaya Bay (early 1950s), and, finally, (d) appearance of a subaerial delta following delivery of coarse sediment to Atchafalaya Bay (early 1970s). Detailed bathymetric surveys, as well as estimates of new land areas using Landsat data, confirm rapid subaerial growth of the Atchafalaya delta since 1972. Bathymetric data taken in 1977 indicate that approximately 15.8 sq mi (40.9 sq km) of Atchafalaya Bay is now occupied by subaerial delta (above the low-tide level). The subaerial phase of development is dominated by deposition of coarse sediment. In this dynamic setting the coarsest sediment particles available to the Lower Atchafalaya River (fine sand) are carried as suspended load during floods. During these flood periods rapid accretion of subaerial delta lobes occurs. Abnormally high flood discharges during the period 1972-75 played an important role in the initiation and rapid evolution of the subaerial delta phase. Major environments of deposition and sedimentary deposits that make up this deltaic sequence are analogous to those that form subdelta components of the modern bird-foot delta of the Mississippi River. The chronology of subaerial development suggests that reentrainment and redeposition of scoured lake and channel-fill deposits during peak floods are linked to the rapid growth of the subaerial distributary-mouth bars documented in this paper.